ProjectM

ProjectM is an open source music visualizer based on the Milkdrop plugin for Windows/Winamp. It now has a Qt GUI that can visualize your audio output through either JACK or PulseAudio, in addition to a libvisual component. This wiki currently focuses on the PulseAudio standalone GUI.

Troubleshooting

The first PulseAudio sound output device is used from the list shown by the command

pacmd list-sources

If this is not the device you want to use, the easiest thing to do is to press 'm' to bring up the menu, then Settings > Pulse Audio Settings. Uncheck the checkbox at the bottom which says "Try first available playback monitor," then select the correct device. Most likely it is one which ends in ".monitor" (if you want the visualizer to visualize what's being output/played).

Some information on usage and configuration can be found in the ProjectM FAQ